1. Field of the Invention
The present invention relates to methods and apparatus for compensating for dispersion in optical communications systems, and in particular to methods and apparatus employing optical phase conjugation.
2. Related Art
In order to have a high transmission capacity, an optical communications system must have low dispersion, this means that pulses of light travelling along the waveguide, generally an optical fibre, of the optical communications system should not suffer significant distortion. This distortion may arise from a number of sources. If the optical communications system employs multi-mode fibre, each of the different modes will have a different group velocity, thus modulated signals, i.e. pulses of light passing down the multi-mode optical fibre, which are made up of a number of different modes of the waveguide will experience a different group delay from each of their modes. This causes a pulse formed from more than one mode to spread out as it propagates, and is called intermodal dispersion. Once consecutive pulses have spread out so that they are no longer distinguishable, one from the other, the information transmission limit of the optical communications system has been reached. This limit is expressed as a bandwidth distance product since it will be reached at a higher bit rate for a shorter optical communications link. Intermodal dispersion between the modes of multi-mode fibres is one of the reasons why modern optical communications systems have moved to the use of single mode optical fibre which, since it only supports one optical mode, does not suffer from intermodal dispersion.
However single mode optical communications systems do suffer from pulse spreading due to the small, but finite bandwidth of the optical source employed. This type of pulse spreading is called chromatic dispersion, and is due to two effects. Firstly, material dispersion is present because the refractive index of a dispersive medium, such as silica from which optical fibres are typically made, depends on wavelength. Secondly, waveguide dispersion, since the propagation characteristics of the single mode supported by a single mode fibre also depend on wavelength. Since the material dispersion of silica is positive at most wavelengths of interest for optical communications systems, and the waveguide dispersion for single mode fibres is negative, these two effects can be carefully balanced in a well designed optical fibre so as to Vive zero total, chromatic dispersion at the operating wavelength of the optical communications system.
The vast majority of optical communications systems which have been installed worldwide contain single mode optical fibre which has been designed for use in the 1.3 .mu.m low loss window, and as such has low chromatic dispersion at this wavelength. In recent years the rapid development of erbium doped fibre amplifiers (EDFA) has meant that fibre loss, and thus the power budget of optical communications systems, is no longer the fundamental limit to achievable transmission distance. However these EDFAs are only operable in the 1.55 .mu.m optical transmission window so that if existing optical communication links are to be upgraded, for example to operate at higher bit rates, these systems must operate in the 1.55 .mu.m window, over optical fibre designed for use at 1.3 .mu.m. Thus the fundamental bandwidth distance product transmission limit when upgrading an existing optical communications system is that imposed by dispersion. Furthermore, even for systems having fibre designed for use at 1.55 .mu.m, as very high bit rates are approached, unless very narrow linewidth, externally modulated lasers are employed, dispersion again is the fundamental limit to transmission capacity.
A number of methods of compensating for dispersion are known. In one such technique the optical signal, at the transmitter end of the optical communications system, is deliberately distorted before being launched into the optical fibre. The distortion imposed upon the optical signal must be calculated so as to be compensated by the dispersion that the optical signal subsequently suffers during propagation along the optimal fibre. An example of such a technique is that disclose& by Koch and Alferness in "Dispersion Compensation By Active Predistorted Signals Synthesis" Journal of Lightwave Technology, volume LT-3, no. 4, August 1985. In order to successfully apply these techniques the transmission characteristics of the particular optical fibre employed, and the length of the transmission link need to be known so that the predistorted signal can be correctly synthenised. Generally the optical source employed in these systems needs to be sophisticated, and thus complex, so at to allow independent control of the amplitude and frequency of the optical signal. The problems inherent in predistortion dispersion compensation systems are considerably exacerbated for higher bit rate systems, where indeed dispersion compensation is of greatest importance.
In a second, known dispersion compensation technique a negative dispersion optical fibre is employed to compensate either at the transmission end, or at the receiver end of the optical link for the positive dispersion suffered by optical signals propagating along the transmission optical fibre. When optical signals at 1.55 .mu.m are transmitted along a transmission optical fibre having a dispersion zero at 1.31 .mu.m, the signals will suffer positive dispersion i.e. the sign of the differential of their group delay with wavelength, will be positive, and will typically be of the order of 17 ps/km/nm. Single mode optical fibre can be specifically designed to have a large negative chromatic dispersion, by choosing the waveguide parameters to give large negative waveguide dispersion, for example a fibre having a core of small diameter and high refractive index will have negative waveguide dispersion. Such a scheme was employed by Izadpanah et al in "Dispersion Compensation In 1310 nm Optimised SMFs Using Optical Equaliser Fibre, EDFAs And 1310/1550 nm WDM" Electronics Letters, 16 Jul. 1992, volume 28, no. 15, page 1469. Izadpanah et al employed a specially designed negative dispersion fibre having a dispersion of -45 ps/km/nm. The length of negative dispersion fibre required was approximately one third of the length of the transmission link over which dispersion was being compensated. Such large lengths of additional fibre are clearly inconvenient and expensive. Furthermore due to the high level of doping used in the core, and the small core size this fibre had a relatively high loss, so that amplification of the optical signal is essential, even if the bit rate of the system is not increased.
A third form of dispersion compensation has been theoretically proposed by Yariv et al in "Compensation For Channel Dispersion By Non-linear Optical Phase Conjugation" Optics Letters, volume 4, no. 2, February 1979. Yariv et al proposed that by generating an optical phase conjugate replica of the optical signal after it has passed through one half of the optical transmission link, and launching this phase conjugate replica into the second half of the optical transmission link, the effects of the dispersion suffered by the optical signal in the first half of the link will be reversed and the optical signal will be restored to its original shape. This technique relies on the time inversion of the group velocity dispersion of the optical signal caused by phase conjugation, and thus requires that the dispersion in the second half of the optical transmission link is the same as the dispersion in the first half, it it is to be fully compensated for.
Yariv's proposal has been implemented in an optical communications system by employing non-degenerate four wave mixing (NDFWM) in dispersion shifted fibre (DSF) to provide the necessary optical phase conjugation. In this case the phase conjugate optical signals travel in the same direction as the copropagating pump light and original optical signal. This technique has been demonstrated by Watanabe et al in "Compensation Of Chromatic Dispersion In A Single Mode Fibre BY Optical Phase Conjugation" IEEE Photonics Technology Letters, volume 5, no. 1, January 1993 and by Jopson et al in "Compensation Of Fibre Chromatic Dispersion By Spectral Inversion" Electronics Letters, 1 Apr. 1993, volume 29, no. 7. In both cases long lengths, over 20 km, of carefully designed DSF were required. A DSF is a fibre which has been designed to have zero dispersion in the 1.55 .mu.m telecommunications window, i.e. its dispersion zero has been shifted from 1.3 .mu.m to around 1.55 .mu.m. In addition to this requirement, Watanabe and Jopson furthermore needed to arrange for the wavelength of the pump light required for NDFWM in the DSF to be the same as the zero dispersion wavelength of the DSF, in order to achieve sufficient phase matching between the pump and the optical signals. This requirement for phase matching over the 20 km of the DSF is severe, and means that the wavelength of the pump light must be carefully controlled e.g. over 20 km with a pump and signal separation of 2 nm the pump wavelength must be within approximately 1 nm of the dispersion zero wavelength. Furthermore this requirement becomes more severe as the length of the DSF increases, and rapidly more severe as the wavelength separation of the pump and signal is increased. The efficiency of conversion of the optical signal to its phase conjugate replica achieved by both Watanabe and Jopson is low, approximately -25 dB in both cases. Thus the phase conjugate signal to be launched into the second half of the optical transmission link is at a very low level.
It has been suggested by Murata et al in "THz Optical Frequency Conversion Of 1 Gb/s Signals Using Highly Non-degenerate Four Wave Mixing In An InGaAsP Semiconductor Laser" IEEE Transactions Photonics Technology Letters, volume 3, no. 11, November 1991, that Yariv's phase conjugation technique for dispersion compensation could be implemented by employing a semiconductor Fabry-Perot laser as the phase conjugating device. This suggestion has not, however been demonstrated.
Another known technique for dispersion compensation is described in "Chirping Compensation Using a Two-Section Semiconductor Laser Amplifier"--Journal of Lightwave Technology, vol. 10, no. 9, September 1992, pages 1247-1254.